Generally, a construction machine such as a hydraulic excavator is constituted by an automotive lower traveling structure, an upper revolving structure which is swingably mounted on the lower traveling structure, and a working mechanism liftably mounted on the front side of the upper revolving structure. The upper revolving structure has an engine mounted on the rear side of a revolving frame to drive a hydraulic pump, and has a cab, a fuel tank, an operating oil tank, and the like mounted on the front side of the revolving frame.
Generally, a diesel engine is used as the engine of a hydraulic excavator. It is known that the diesel engine emits harmful substances such as particulate matter (PM) and nitrogen oxides (NOx), and the like. Accordingly, the hydraulic excavator is so constructed as to be provided with an exhaust gas treatment apparatus in an exhaust pipe for forming an exhaust gas passage of the engine.
This exhaust gas treatment apparatus has treatment members such as a particulate matter removing filter (generally called a diesel particulate filter, abbreviated as DPF) for capturing and removing the particulate matter in the exhaust gas, a selective reduction catalyst which purifies nitrogen oxides (NOx) by use of a urea water solution, and an oxidation catalyst for oxidizing and removing carbon monoxide (CO) and hydrocarbons (HC) (see, for example, Patent Literature 1: Japanese Patent Laid-Open No. 2003-120277 A).
Here, the exhaust gas treatment apparatus according to Patent Literature 1 is comprised of, for example, an upstream cylinder disposed on an upstream side in the flowing direction of exhaust gas of an engine, a downstream cylinder disposed on the downstream side, and a purifying part cylinder provided in series between these cylinders. And treatment members, including a particulate matter removing filter, an oxidation catalyst, a urea water injection valve, a selective reduction catalyst, and the like, are accommodated in this purifying part cylinder. The upstream cylinder, the purifying part cylinder, and the downstream cylinder respectively have flange-like connecting surfaces substantially perpendicular to the axial direction of the cylinders (longitudinal direction of the cylinders), and are fastened together by using fasteners in a state in which these connecting surfaces are faced to each other with a gasket or the like interposed therebetween. Thus, the exhaust gas treatment apparatus is so constructed as to be mounted on a structure such as the engine provided on the upper revolving structure in the state in which the purifying part cylinder is connected between the upstream cylinder and the downstream cylinder in series in the axial direction.